1. Field of the Invention
This invention relates to an automotive vehicle steering apparatus and more particularly to an apparatus employing a hydraulic actuator for steerable road wheels.
2. Description of the Prior Art
Japanese published unexamined utility model application No. 56-136855 discloses the automotive vehicle steering apparatus shown in FIG. 1. This steering apparatus comprises an oil pump 1 driven by an engine, a reservoir 2, a control valve 3, a power cylinder 4 for actuating steerable road wheels, an electrical contact unit 5 for the control valve 3, and connecting hydraulic lines. The control valve 3 includes a spool-type valve body 3a and two solenoids 3b electrically connected to the control unit 5. The control unit 5 receives electrical signals T, .omega., .omega. and V and provides the solenoids 3b with controlled electrical currents in response thereto. The signal T represents a manual steering force applied to a steering wheel, not shown. The signal .omega. represents the angular deviation of the steering wheel from its normal position. The signal .omega. represents the angular velocity of the steering wheel. The signal V represents a vehicle speed.
This automotive vehicle steering apparatus has a problem in that, since the steerable road wheels are simply hydraulically controlled through the control valve 3, the automotive vehicle cannot be adequately steered if the control unit 5 and/or any of sensors outputting the electrical signals T, .omega., .omega. and V malfunction.